Gluten development
Gluten development is the formation of an elastic protein network when the glutenin and gliadin in wheat flour are hydrated and worked, which gives dough strength and stretch but makes pastry tough if taken too far.
Bread wants a strong network, pastry wants a weak one. Fat rubbed into the flour coats the proteins and blocks the network from forming, which is why pastry recipes insist on minimal water, minimal mixing, and handling the dough as little as possible.
Some gluten is still necessary, or the pastry would fall apart when rolled and lifted. Resting the dough lets whatever gluten has formed relax, so a well-rested dough rolls out without springing back and bakes without shrinking.